Distortion in one channel Mcintosh 225 amplifier

[nvidiagal01]

I've had a Mcintosh 225 tube amplifier for about 5 years and the other day it developed distortion in one channel when the volume was turned up fairly loud. Here's what I've done: 1) Verified bias is at -21 volts for all 4 output tubes 2) Replaced all tubes with known good ones one-at-a-time 3) Verified all voltages per the maintenance manual 4) Replaced all coupling caps 5) Cleaned and De-Oxed all tube sockets and trim pots 6) Checked and replaced any resistor not to spec. I'm at a loss what to do next. The tubes don't glow fiery red and the fuse doesn't blow. I'm not thinking power supply problem because each channel draws from the same power source and the filter caps are only 5 years old. It couldn't be output transformer problem could it?

 

[knockbill]

Sounds like a one channel problem, does it have the distortion with any input (CDP, for instance) or just one? Possibly dirty selector switch, or pot on that side,,, or bad/loose input cable... check the speaker cables/connections also... I always start with the easy stuff...

 

[dshoaf]

Next step is to take voltage readings for each stage. You'll need to compare the plate, grid and cathode readings you get to the schematic's voltages to identify the offending stage.

First ID the channel and then start with the 12AX7 and work down the signal chain, stage by stage, collecting voltage readings. Make a copy of the schematic and write the voltage readings you take next to the voltages printed in the schematic.

Once complete, sit back and look for where there's a significant variance. When you've got those identified, then kill the power and let the caps discharge. Remove the tubes for that offending channel and then test each resistor related to the out of spec readings. I'll bet you're going to find resistors out of spec.

Check any other resistors for hair line cracking. The are created when you stress the board as the caps were replaced. In other words, you may have introduced other problems as you did the restoration work.

Hope that gets you started.

Cheers,

David

 

[peterh]

I've had a Mcintosh 225 tube amplifier for about 5 years and the other day it developed distortion in one channel when the volume was turned up fairly loud. Here's what I've done: 1) Verified bias is at -21 volts for all 4 output tubes 2) Replaced all tubes with known good ones one-at-a-time 3) Verified all voltages per the maintenance manual 4) Replaced all coupling caps 5) Cleaned and De-Oxed all tube sockets and trim pots 6) Checked and replaced any resistor not to spec. I'm at a loss what to do next. The tubes don't glow fiery red and the fuse doesn't blow. I'm not thinking power supply problem because each channel draws from the same power source and the filter caps are only 5 years old. It couldn't be output transformer problem could it?

You already have broken the golden rule of fault diagnostic. Send it to a qualified technician before it's too late.

I would have done :
- check voltages ( as you did )
- connected external load resistor and a source of sinewave
started the amp and followed the signal chain with a scope to identify where changes between channels occur.
-when one or a few diagnoses are made, figure out how to confirm ( may be tube swap or measuring
a component ) If a defective component is found, replace that component and retest. Don't swap things wild you will
risk introducing even more faults.

 

[dcgillespie]

Is there any chance that it is not the amplifier? That is, has the offending channel's speaker developed a problem? No doubt you've swapped input sources, but I'd confirm that the speakers are not an issue as well, particularly because you indicate that it only happens at high volume.

Dave